A nation-wide crackdown on online gambling in the Philippines has led to the detention of more than a thousand Chinese nationals, raising concerns from China’s Foreign Ministry and potentially straining ties between the two countries.

1200 Chinese workers were operating in call center-like facilities in former U.S. air base, Clark Field, where the Philippines immigration bureau believed an online gambling operation was being run.

Many of the Chinese nationals are thought to be working in the Philippines illegally.

Gen Shuang, Chinese Foreign Ministry spokesman, said: “China expresses concern over the large number of Chinese citizens detained by the Philippines.” But also noted that China had requested the Philippines make arrangements for those who had been detained and to release those with legal identification.

It’s unknown if the Chinese nationals are being held for gambling or immigration violations. Gambling is legal in the country but only if permitting and zoning regulations are followed.

China has supported the Philippines' other law enforcement efforts, including President Rodrigo Duterte's anti-narcotics campaign, which has drawn wide-spread condemnation from Western countries over extrajudicial killings.

Duterte has made efforts prior to this incident to improve historically strained ties with China, which have been plagued by territorial disputes in the South China Sea, making this a surprising move by the Philippines.